This invention relates to an improved process for preparing composite materials by coating a powder of an inorganic substance with a polymer.
Various methods for coating powdery inorganic materials with organic polymers by means of in situ polymerization have heretofore been proposed. For example, U.S. Pat. No. 3,330,693 discloses a method for coating the surface of iron oxide powder with polyethylene in an aqueous medium, and British Pat. No. 1,156,653 discloses a method for coating the surface of red iron oxide, indigo red or titanium oxide powder with a copolymer of methyl methacrylate and methacrylic acid in petroleum ether. Further, Japanese Pat. No. 302,461 discloses a method for preparing compositions comprising a pigment and a polymeric substance by polymerizing a monomer in the presence of various kinds of pigments with the aid of a catalytic system.
In each of these known methods, however, the polymerization of the monomer proceeds predominantly in the reaction medium rather than on the surface of the particles of the inorganic compound and, therefore, the amount of the polymer actually coated on the particles is very small and the bulk of the polymer does not participate in the coating. In short, each of these known methods is defective in that the ratio of the coated polymer to the total of the available polymer is very low.